emacs: use lexical-bindings in all libraries

emacs: use lexical-bindings in all libraries

Doing so causes many new compile warnings.  Some of these warnings
concern genuine changes in behavior that have to be addressed right
away.

Many other warnings are due to unused variables.  Nothing has changed
here, except that the byte-compiler can now detect these pre-existing
and harmless issues.  We delay addressing these issues so that we can
focus on the important ones here.

A third group of warnings concern arguments that are not actually used
inside the function but which cannot be removed because the functions
signature is dictated by some outside convention.  Silencing these
warning is also delayed until subsequent commits.